BC1 Hybrids Research Articles

Drought resistance of Trifolium repens×Trifolium uniflorum interspecific hybrids

White clover (Trifolium repens L.) is a widely used and highly valued temperate legume; however, its productivity and survival are restricted under dryland and drought conditions. This study investigated whether drought resistance of white clover could be improved by interspecific hybridisation with Trifolium uniflorum L. After almost 4 months without irrigation in a rain-shelter facility, shoot dry weight (DW) decreased significantly less in first-generation backcross (BC1) hybrids (–47%) than second-generation backcross (BC2) hybrids (–68%) and white clover (–69%). Stolon morphological parameters such as internode length and leaf lamina area also decreased less under water stress in the BC1 hybrids than in BC2 and white clover. There was also lower senescence in BC1 under water stress than in the other clover types. Genotypes with smaller changes in leaf lamina area, internode length, senescence and lateral spread had smaller changes in shoot DW, and there were significant correlations between constitutive levels of some characteristics and the effect of water stress on shoot DW. Under water stress, the growth form of the BC1 hybrids was compact, dense and prostrate, whereas white clover was more spreading and open. Increased allocation of dry matter to roots under drought, and greater root diameter, may also have influenced the ability of BC1 hybrids to maintain water uptake and key physiological processes. Overall, the data confirm that the drought resistance of white clover can be improved through hybridisation with T. uniflorum.

Production and genetic characterization of interspecific hybrids among Crambe abyssinica, Crambe hispanica and Crambe kralikii.

In this paper, interspecific crosses among Crambe abyssinica, Crambe hispanica, and Crambe kralikii were reported. In the C. hispanica x C. abyssinica (H x A) cross, 118 F1 hybrids were produced without embryo rescue, while 5 F1 hybrids were obtained with embryo rescue, when C. hispanica was used as the female parent. In the reciprocal cross (A x H), 232 hybrids were obtained without embryo rescue. From more than 1000 C. kralikii flowers pollinated with pollen grains of C. abyssinica (K x A), only 2 F1 hybrids were obtained with embryo rescue, whereas the reciprocal cross produced no hybrids, even with embryo rescue. The hybrids were confirmed at the morphological, cytological, and molecular levels. In the combinations of A x H and H x A, many BC1 hybrids were obtained without embryo rescue. In contrast, in the K x A cross, only 7 BC1 plants were obtained with embryo rescue, while no seed set was achieved under self-pollination or in backcrosses without embryo rescue. In the H x A F1 hybrids, the pollen stainability was 65.4-86.0%, with an average of 76.9%. In comparison, the pollen viability of hybrids in the reciprocal cross (A x H) ranged from 66.2 to 81.1%, with an average of 75.4%. Fertile pollen grains were not found in the K x A F1 hybrids. All F1 hybrids of the 3 crosses (H x A, A x H, and K x A) had the expected 2n = 75 chromosomes. AFLP analyses indicated that all F1 hybrids and their progenies had typical bands of the parents. These hybrids and progenies are anticipated to be valuable for future C. abyssinica improvement in breeding programs.

Crop to wild introgression in lettuce: following the fate of crop genome segments in backcross populations.

BackgroundAfter crop-wild hybridization, some of the crop genomic segments may become established in wild populations through selfing of the hybrids or through backcrosses to the wild parent. This constitutes a possible route through which crop (trans)genes could become established in natural populations. The likelihood of introgression of transgenes will not only be determined by fitness effects from the transgene itself but also by the crop genes linked to it. Although lettuce is generally regarded as self-pollinating, outbreeding does occur at a low frequency. Backcrossing to wild lettuce is a likely pathway to introgression along with selfing, due to the high frequency of wild individuals relative to the rarely occurring crop-wild hybrids. To test the effect of backcrossing on the vigour of inter-specific hybrids, Lactuca serriola, the closest wild relative of cultivated lettuce, was crossed with L. sativa and the F1 hybrid was backcrossed to L. serriola to generate BC1 and BC2 populations. Experiments were conducted on progeny from selfed plants of the backcrossing families (BC1S1 and BC2S1). Plant vigour of these two backcrossing populations was determined in the greenhouse under non-stress and abiotic stress conditions (salinity, drought, and nutrient deficiency).ResultsDespite the decreasing contribution of crop genomic blocks in the backcross populations, the BC1S1 and BC2S1 hybrids were characterized by a substantial genetic variation under both non-stress and stress conditions. Hybrids were identified that performed equally or better than the wild genotypes, indicating that two backcrossing events did not eliminate the effect of the crop genomic segments that contributed to the vigour of the BC1 and BC2 hybrids. QTLs for plant vigour under non-stress and the various stress conditions were detected in the two populations with positive as well as negative effects from the crop.ConclusionAs it was shown that the crop contributed QTLs with either a positive or a negative effect on plant vigour, we hypothesize that genomic regions exist where transgenes could preferentially be located in order to mitigate their persistence in natural populations through genetic hitchhiking.

INTROGRESSION BREEDING IN GENUS TULIPA ANALYSED BY GISH

INTROGRESSION BREEDING IN GENUS TULIPA ANALYSED BY GISH

Interspecific cross of Brassica oleracea var. alboglabra and B. napus: effects of growth condition and silique age on the efficiency of hybrid production, and inheritance of erucic acid in the self-pollinated backcross generation

Interspecific hybrids were produced from reciprocal crosses between Brassica napus (2n = 38, AACC) and B. oleracea var. alboglabra (2n = 18, CC) to introgress the zero-erucic acid alleles from B. napus into B. oleracea. The ovule culture embryo rescue technique was applied for production of F1 plants. The effects of silique age, as measured by days after pollination (DAP), and growth condition (temperature) on the efficiency of this technique was investigated. The greatest numbers of hybrids per pollination were produced under 20°/15°C (day/night) at 16 DAP for B. oleracea (♀) × B. napus crosses, while under 15°/10°C at 14 DAP for B. napus (♀) × B. oleracea crosses. Application of the ovule culture technique also increased the efficiency of BC1 (F1 × B. oleracea) hybrid production by 10-fold over in vivo seed set. The segregation of erucic acid alleles in the self-pollinated backcross generation, i.e. in BC1S1 seeds, revealed that the gametes of the F1 and BC1 plants carrying a greater number of A-genome chromosomes were more viable. This resulted in a significantly greater number of intermediate and a smaller number of high-erucic acid BC1S1 seeds.

Comportamiento agronómico del cultivar híbrido RC1 de Palma de aceite (Elaeis oleifera x Elaeis guineensis) x Elaeis guineensis

Se realizó transferencia de genes desde la especie Nolí (Elaeis oleifera) a la especie Palma de aceite (Elaeis guineensis) mediante retrocruzamientos con el propósito de obtener un cultivar que tuviera alta producción de frutos y aceite por unidad de área, baja tasa de crecimiento, tolerancia a enfermedades, y aceite con alto contenido de ácidos grasos insaturados y carotenos. La primera generación de retrocruzamiento RC1 se obtuvo mediante polinización controlada entre palmas del híbrido interespecífico F1 (Elaeis oleifera x Elaeis guineensis) usadas como progenitor femenino, con palmas de la especie Elaeis guineensis como progenitor masculino. En 1995 se establecieron en campo las descendencias de cinco cruces RC1, planteando como hipótesis de segregación que el 50% de los descendientes corresponderían al genotipo de palma de aceite y el 50% restante al genotipo del híbrido RC1 con carga genética africana (75% E. guineensis y 25% E. oleifera). Los datos de campo se analizaron con base en un diseño completamente al azar con arreglo jerárquico y desigual número de repeticiones por tratamiento; para probar la hipótesis se aplicó la prueba de Chi-cuadrado (χ2). En el cuarto año de producción los híbridos RC1 promisorios alcanzaron 35,0 t·ha-1 de fruto, con 19,6% de aceite y una producción anual estimada de aceite de 6,3 t·ha-1. Estos híbridos crecieron en promedio 25,8 cm por año, característica que aumenta a más de 40 años la expectativa de vida útil de la plantación. Hasta la fecha presentan tolerancia a cuatro enfermedades de carácter letal que afectan la Palma de aceite en Colombia, puesto que durante siete años de evaluación no se han presentado síntomas. El aceite de estos híbridos contiene mayor concentración de ácidos grasos insaturados y carotenos que el de la Palma de aceite.

Genetic basis of species differentiation between Coffea liberica Hiern and C. canephora Pierre: Analysis of an interspecific cross

The phenotypic and genetic differentiation between the two related Coffea species (C. liberica Hiern and C. canephora Pierre) was examined. These species differed markedly in terms of leaf, inflorescence, fruit and seed characters. A genetic map of the interspecific cross Coffea liberica × C. canephora was constructed on the basis of 72 BC1 hybrids. Eighty-three AFLP markers, four inter simple sequence repeats (ISSR) and five microsatellites corresponding to Coffea liberica species-specific markers were mapped into 16 linkage groups. The total length of the map was 1502.5 cM, with an average of 16.3 cM between markers and an estimated genome coverage of 81%. The two species were evaluated relative to 16 quantitative traits and found to be significantly different for 15 of them. Eight QTLs were detected, associated with variations in petiole length, leaf area, number of flowers per inflorescence, fruit shape, fruit disc diameter, seed shape and seed length. Results on segregation distortion and the under-representation of particular markers were interpreted in terms of genome differentiation. The implications for the introgression of QTLs involved in advantageous morphological traits (number of flowers per inflorescence, fruit and seed shape) are discussed.

Features of the formation of self-fertile euploid lines (2n = 42) by self-pollination of the 46-chromosome barley-wheat BC1 hybrid Hordeum marinum subsp. gussoneanum Hudson (= H. geniculatum All.) (2n = 28) × Triticum aestivum L. (2n = 42)

We studied some features of the development of self-fertile 42-chromosome lines on the base of self-pollination progeny of 46-chromosome plants obtained by backcrossing of barley--wheat hybrids Hordeum marinum subsp. gussoneanum Hudson (= H. geniculatum All.) (2n = 28) x Triticum aestivum L. (2n = 42). The stabilization of karyotypes, resulting in 42-chromosome plants of the wheat type was generally completed by generation BC1F10. The plants of all self-pollination progenies, including BC1F10, showed some phenotypic traits characteristic of wild barley. Plants of BC1F10 with the chromosome sets 2n = 42 and 2n = 42 + t were analyzed by RAPD with a set of 115 primers. Fragments of the wild barley genome were detected in RAPD patterns with 19 primers. Cross-hybridization confirmed that these fragments belonged to the wild barley genome. We raised four phenotypically different 42-chromosome lines from grains obtained from plants of generation BC1F10, and these lines proved to be cytogenetically stable and self-fertile when grown in the field.

Adaptive Introgression of Herbivore Resistance Traits in the Weedy SunflowerHelianthus annuus

The role of hybridization in adaptive evolution is contentious. While many cases of adaptive trait introgression have been proposed, the relevant traits have rarely been identified, resulting in a lack of clear examples of this process. Here, we examine a purported case of adaptive introgression in which the annual sunflower Helianthus annuus annuus has captured alleles from a congener (Helianthus debilis) to form a stabilized hybrid, Helianthus annuus texanus. We tested the hypotheses that herbivore resistance traits have introgressed from H. debilis to H. annuus and have increased adaptation in the latter. In two common gardens, fitness (estimated by seed production) was on average 55% higher in H. a. texanus than in H. a. annuus. For H. a. texanus, three damage traits (of seven tested) differed significantly from the H. a. annuus parent in one or both sites and were shifted in the direction of the more resistant H. debilis. Natural selection favored H. a. annuusxH. debilis BC(1) hybrids (synthesized to mimic the ancestors of H. a. texanus) with H. debilis-like resistance to seed midges Neolasioptera helianthis and to receptacle/seed feeding Lepidoptera at one or both sites. Assuming similar herbivore pressures in the past, these results suggest that introgression of biotic resistance traits was important in the adaptation of H. annuus to central and southern Texas.

Inheritance of thermo-photoperiod sensitive male sterility in wheat

The fertility of a wheat male sterile line 337S was investigated in 4 consecutive years with 18 different sowing dates. Line 337S showed high sterility under both short daylength/low temperature and long daylength/high temperature during ear development. It has 2 time windows to be used as a male sterile line for hybrid seed production. Its fertility rate can be >50% with suitable sowing time; thus, it can be self-maintained as a male sterile line. Line 337S was reciprocally crossed with 7 common wheat varieties and the fertility of their F1, F2, and BC1 hybrids was investigated at different sowing dates. The results showed that recessive nuclear genes controlled male sterility in 337S and no cytoplasmic effect was observed. All common wheat varieties were able to restore its fertility. The male sterility was controlled by a pair of recessive genes under short daylength/low temperature, but was governed by 2 pairs of recessive genes under long daylength/high temperature. This novel male sterile line provides a new tool for using heterosis in wheat.

Adaptive Introgression of Herbivore Resistance Traits in the Weedy Sunflower Helianthus annuus

The role of hybridization in adaptive evolution is contentious. While many cases of adaptive trait introgression have been proposed, the relevant traits have rarely been identified, resulting in a lack of clear examples of this process. Here, we examine a purported case of adaptive introgression in which the annual sunflower Helianthus annuus annuus has captured alleles from a congener (Helianthus debilis) to form a stabilized hybrid, Helianthus annuus texanus. We tested the hypotheses that herbivore resistance traits have introgressed from H. debilis to H. annuus and have increased adaptation in the latter. In two common gardens, fitness (estimated by seed production) was on average 55% higher in H. a. texanus than in H. a. annuus. For H. a. texanus, three damage traits (of seven tested) differed significantly from the H. a. annuus parent in one or both sites and were shifted in the direction of the more resistant H. debilis. Natural selection favored H. a. $$annuus\times H.$$ debilis BC1 hybrids (synthesized to mimic the ancestors of H. a. texanus) with H. debilis–like resistance to seed midges Neolasioptera helianthis and to receptacle/seed feeding Lepidoptera at one or both sites. Assuming similar herbivore pressures in the past, these results suggest that introgression of biotic resistance traits was important in the adaptation of H. annuus to central and southern Texas.

IMPROVEMENT OF BREEDING EFFICIENCY FOR INTERSPECIFIC HYBRIDIZATION OF LILIES IN KOREA

Interspecific hybridization of lily in Korea is focused on increasing breeding efficiency especially under Korean climate conditions. Cultivars derived from L. formolongi were propagated by seed and were sensitive to day length. These cultivars differ from flowering time with the Asiatic and Oriental hybrids. Therefore, to breed interspecific hybrids by using L. formolongi, the use of stored viable pollen is a prerequisite. Lily pollen was stored at four different temperature conditions for one year and pollen viability was investigated using three different methods. As storage temperatures increased, pollen viability was drastically reduced. Viability of lily pollen stored was maintained effectively at temperatures lower than -20°C. FDA test was the most effective and convenient method to ascertain pollen viability. To overcome pre-fertilization barriers in interspecific hybridization of lilies, pollen tube growth in the stigma or style, and the penetration of pollen tubes into the ovules were observed. OA (Oriental Asiatic), OL (Oriental Longiflorum), FA (Formolongi Asiatic), and FO (Formolongi Oriental) hybrids were obtained through embryo rescue and stigmatic and CSM pollination, while AO (Asiatic Oriental), AL (Asiatic Longiflorum), LA (Longiflorum Asiatic) and LO (Longiflorum Oriental) hybrids were not obtained through these methods. Backcrosses were used in order to reduce scattered petals in interspecific hybrids. It was possible to obtain BC107 hybrids between FA (Fomolongi Asiatic) and Asiatic hybrids as male parent. But, it was difficult to obtain BC1 hybrids between FA hybrid and L. formolongi as female parent. Normal pollination was better than CSM (Cut style pollination method) to get BC1 hybrids. In order to overcome the sterility of interspecific hybrids pollen, low concentration of oryzalin was more effective than high concentration of colchicine to obtain tetraploid. The fertility of pollen increased by about 40% in tetraploid level. Fusarium oxysporum screening test was carried out. ‘Hae-wool’, ‘Sinavro’ and FA 96-16 were classified as highly disease resistant and ‘Supia’ and ‘Doran’ as moderately resistant to Fusarium bulb-rot. The FO-hybrid lilies from NHRI were susceptible.

Male and female gametophyte selection of barley for salt tolerance.

The contribution of the gametophyte in the inheritance of salt tolerance was studied by crossing F3 and BC1 hybrids of the tolerant cultivars Rannii 1 and Pirkka with the sensitive cultivar K-30356. The third generation shows that the progenies of heterozygous plants grown in saline conditions show elevated salinity tolerance. Again, a comparison of the BC1 hybrid progenies shows that the male and female gametophytes contribute to the inheritance of salt tolerance. Gametic selection is maximally efficient during the formation of the female gametophyte and the germination of pollen grains on the stigma.

Comparative fitness of a wild squash species and three generations of hybrids between wild×virus-resistant transgenic squash

We compared some fitness components of the wild squash species Cucurbita pepo spp. ovifera var. texana (C. texana) and three generations of hybrids (F1, BC1, and BC2) between C. texana and commercial transgenic squash CZW-3 over three consecutive years under field conditions of low (LDP) and high disease pressure (HDP) by Cucumber mosaic virus (CMV), Zucchini yellow mosaic virus (ZYMV) and Watermelon mosaic virus (WMV). Transgenic squash CZW-3 expresses the coat protein (CP) genes of CMV, ZYMV, and WMV, and is resistant to these three aphid-borne viruses. Across all HDP trials, transgenic BC1 and BC2 hybrids expressing the three CP genes grew more vigorously, displayed resistance to CMV, ZYMV, and WMV, and produced a greater number of mature fruits and viable seeds than nontransgenic hybrid segregants and C. texana. Transgenic F1 hybrids behaved similarly to BC1 and BC2 hybrids but grew less vigorously than C. texana. In contrast, across all LDP trials, C. texana outperformed the transgenic and nontransgenic hybrid segregants. Further, only one back cross was necessary to recover individuals with most of the C. texana characteristics and yet maintain virus resistance. Our data suggest that C. texana acquiring CP transgenes upon hybridization and introgression could have a selective advantage if CMV, ZYMV, and WMV are severely limiting the growth and reproductibility of wild squash populations.

Alleles of diabetes-resistant BN rats contribute to insulin-dependent type 1 diabetes mellitus

Diabetes in the biobreeding (BB) rat results from autoimmune destruction of pancreatic β cells and thereby it is sharing many features with human type 1 diabetes. Independent crossing studies have demonstrated that diabetes in the BB rat is explained by at least three recessively acting genes termed Iddm1 (major histocompatibility complex), Iddm2 (lymphopenia), Iddm3 (unknown). About 50% of Iddm1 and Iddm2 homozygous first backcross hybrids (BC1) usually develop diabetes. However, 75% of these homozygotes become diabetic when using diabetic BB/HRI and diabetes-resistant BN/Mol rats. That prompted us to carry out a cross between BB/OK and BN/Crl rats in order to localise diabetogenic gene(s) of BB and/or BN rats. Fifty nine Iddm1 and Iddm2 homozygous [(BN×BB)F1×BB] BC1 hybrids (35 M, 24 F) were observed for diabetes occurrence up to an age of 30 weeks. All hybrids were used in a genome-wide scan carried out with 238 microsatellite markers covering about 92% of the genome. Significantly more Iddm1 and Iddm2 homozygous BC1 hybrids became diabetic (69 vs. 50%, p<0.003) with an age at onset of 91±31 days. Significant deviations from expected allele distribution between diabetic and non-diabetic BC1 hybrids were found at loci on chromosomes 1, 2, 3, 9, 10, 15, 16 and 19, with the strongest effect observed at locus D10Mgh2, where more heterozygous (91%) than homozygous diabetics (44%) were found. We conclude that BN rats possess more than one gene contributing to type 1 diabetes development.

Production of Intergeneric Hybrids between the C3-C4 Intermediate Species Diplotaxis tenuifolia (L.) DC. and Raphanus sativus L.

In intergeneric crossings between Diplotaxis tenuifolia (2n = 22, DtDt) and five cultivars of Raphanus sativus (2n = 18, RR), an intergeneric F1 hybrid was produced from the crossing of D. tenuifolia × R. sativus cv. ‘4-season leaf’ through ovary culture followed by embryo culture. The induced amphidiploid (2n = 40, DtDtRR) showed well-regulated meiotic features at PMCs and a high pollen fertility (75 %). Three BC1 hybrids with DtRR (2n = 29) or DtDtR (2n = 31) genome constitutions were obtained by the same embryo rescue procedure in the crossings of amphidiploid × R. sativus and D. tenuifolia × amphidiploid, respectively. In the successive backcrossings of two BC1 hybrids (DtRR, 2n = 29) to R. sativus, 102 BC2 hybrids were obtained by conventional pollination. In the reciprocal crossing of R. sativus × BC1 hybrids, 12 reciprocal BC2 hybrids were also produced without embryo rescue. The somatic chromosome number of 89 BC2 hybrids with D. tenuifolia cytoplasm and 12 reciprocal BC2 hybrids with R. sativus cytoplasm ranged from 2n = 18 to 2n = 23 that were estimated to carry 2n = 18 chromosomes of R. sativus and zero to five chromosomes of D. tenuifolia. Among them, 24.7 % of the BC2 hybrids and 41.6 % of the reciprocal BC2 hybrids were assumed to be monosomic addition lines (MALs, 2n = 19). The novel intergeneric hybrids obtained in this study could become useful materials for investigating the genetic effects on C3-C 4 intermediate traits at the genomic and chromosomal levels, as well as for estimating the performance of genetic improvement in Brassicaceae.

Introgression of chromosomes of Festuca arundinacea var. glaucescens into Lolium multiflorum revealed by genomic in situ hybridisation (GISH)

An F1 hybrid (n=4x=28) between the tetraploid species Festuca arundinacea var. glaucescens (GGG′G′) and a synthetic tetraploid Lolium multiflorum (LmLmLmLm) was backcrossed to diploid L. multiflorum to produce triploid (2n=3x=21) BC1 hybrids (LmLmG). At metaphase I of meiosis the triploids had a preponderance of ring bivalents and univalents with some linear and frying-pan trivalents. Genomic in situ hybridisation (GISH) differentiated the Festuca chromosomes from Lolium and revealed that the bivalents were exclusively between Lolium homologues, while the univalents were Festuca. Despite the limited amount of homoeologous chiasmata pairing in the triploids, some recombinant chromosomes were recovered in the second backcross when the hybrids were further crossed to diploid L. multiflorum. The progeny from the second backcross was predominantly diploid. Genotypes with recombinant chromosomes and chromosome additions involving an extra Festuca chromosome were identified using GISH. Changes in plant phenotype were related to the presence of Festuca chromatin.

MNU Induction of Neoplasia in a Platyfish Model

Interspecific hybrid crosses between members of the fish genus Xiphophorus have been used for over 70 years to study the genetic aspects of melanoma formation. In the well-established “Gordon-Kosswig” cross, the platyfish X. maculatus is outcrossed to the swordtail X. helleri, and the resulting backcross segregants spontaneously develop melanoma. We recently produced a distinct cross between X. maculatus and another platyfish species, X. couchianus. X. maculatus strain Jp 163 A is homozygous for several X-linked pigment pattern genes, including the Spotted dorsal (Sd), Dorsal red (Dr), and Anal fin spot (Af). Af is a sex-limited trait, coding exclusively for melanophores distributed on the modified anal fin or “gonopodium” in the adult male fish. Within F1 and BC1 hybrids (to X. couchianus), the Sd pigment pattern is phenotypically suppressed, whereas Dr and Af are enhanced. We exposed BC1 hybrids to the direct-acting carcinogen N-methyl-N-nitrosourea (MNU). Treatment led to the development of schwannomas, fibrosarcomas, and retinoblastomas. In addition, numerous MNU-treated males that inherited Af developed a pronounced melanotic phenotype, with melanin-containing cells oftentimes totally covering the gonopodium and extending further to grow within the ventral regions of the fish. Genetic linkage analysis of the BC1 hybrids revealed a significant (p < 0.01) association between CDKN2X genotype and the phenotypic degree of melanization. Such an association is consistent with a locus within linkage group V playing a role in the development of melanosis and delineates three genetic preconditions and a carcinogenic scheme resulting in melanosis of the ventral regions of hybrid fish. The overall study further alludes to the potential of using Xiphophorus fish to study carcinogenic mechanisms for tumors other than melanoma (schwannoma, fibrosarcoma, and retinoblastoma) and should enable extensive pathologic and molecular genetic studies of derived neoplastic abnormalities.

Tolerance to low temperatures and tuber soft rot in hybrids between Solanum commersonii and Solanum tuberosum obtained through manipulation of ploidy and endosperm balance number (EBN)

The objectives of this study were to evaluate the tolerance to low temperatures and tuber soft rot in hybrids between Solanum commersonii and Solanum tuberosum. The experimental materials consisted of F1 triploid, BC1 pentaploid-near pentaploid and BC2 tetraploid–near tetraploid hybrids. The F1 triploids had a freezing tolerance and acclimatization capacity closest to S. commersonii. This indicated that the endosperm barriers which prevent the introgression of 1EBN S. commersonii into 4EBN S. tuberosum had been overcome. Indeed, the triploids produced 2n eggs, thus giving a compatible maternal to paternal EBN ratio in the hybrid endosperm generated by the 3x(2EBN) × 4x(3EBN) crosses. The tolerance to low temperatures of BC1 and BC2 hybrids was lower than that of the F1. However, a number of genotypes were identified which were able to withstand temperatures down to -5°C. Some BC2 hybrids were also tested for their tolerance to tuber soft rot, and some resistant hybrids were detected. A number of them combined the capacity for cold acclimatization with tolerance to tuber soft rot. These hybrids have an EBN of 4; they are fertile and have been used in backcrosses with 4EBN S. tuberosum.

Introgression of Lilium rubellum Baker chromosomes into L. longiflorum Thunb.: a genome painting study of the F1 hybrid, BC1 and BC2 progenies.

Interspecific hybrids between Lilium longiflorum (L, 2n = 2x = 24) and Lilium rubellum (R, 2n = 2x = 24) were produced with the aim of transferring desirable horticultural traits from L. rubellum to L. longiflorum. All F1 hybrids (LR, 2n = 2x = 24) and BC1 individuals (LLR, 2n = 3x = 36) were phenotypically uniform for plant height, flowering time, leaf shape and flower colour. The BC1 plants were, in spite of their triploid nature, fertile and could be used as a female parent in backcrossings with autotetraploid L. longiflorum (LLLL, 2n = 4x = 48). Twelve BC2 individuals were obtained and three of them were selected for further chromosome analysis. As L. longiflorum and L. rubellum chromosomes were indistinguishable in the hybrids, genomic in-situ hybridization (GISH) was applied to establish the parentage of the chromosomes of the F1 hybrids and the BC1 and BC2 progenies. GISH confirmed the LLRR constitution of the doubled amphimonoploid (allodiploid), and the LLR constitution of all BC1 plants. The three selected BC2 plants were, as expected, aneuploid, containing three complete sets of L. longiflorum chromosomes and six, seven or eight L. rubellum chromosomes, respectively. However, L/R translocation or recombinant chromosomes could not be demonstrated in the mitotic metaphase complements of the F1, BC1 and BC2 plants. In spite of the high frequencies of homoeologous recombination in the F1 hybrids (LR) pollen was found to be sterile in all cases. At metaphase I of the pollen mother cells of the BC1 plants, genome painting did not reveal any cases of homoeologous pairing and recombination between L and R chromosomes. This lack of exchange between homoeologous chromosome segments indicates complete preferential pairing of the L and R chromosomes in the F1 (amphidiploid) and BC1 plants. It seems that the preferential pairing in the F1 and BC1 hybrids hinder the introgression of the chromosome segments or species-specific genes into the recipient for breeding purposes.